This relates in general to isogrid filament wound composite structures and more specific to the formation of pseudo-isotropic laminate zones in some of the joint regions of the structure.
The isogrid concept consists of a repetitive triangular pattern of ribs with an integral skin. Its name is derived from the term "isotropic" which means that the structure has essentially the same mechanical properties in all directions on a global level. This provides the structure with the unique advantage of not requiring ring frames for compression loaded cylinders (except for large length-to-diameter ratios), thereby reducing part count and assembly operations. Typically, the isogrid structure is either machined from a metal structure or it is produced by chem milling. The unique concept of the invention can be applied to composite material to take advantage of the mass reduction available with composite materials' high specific strength and modules. The isogrid concept is most efficiently produced using filament winding. The ribs of the isogrid ribs are produced by repetitively winding fibers (pre-impregnated or wet) into recesses built into the winding mandrel. After the recesses have been completely filled, skin is filament wound over the top of these recesses. The entire structure is then cured as a unit. After completion of the cure, the segmented mandrel is removed, leaving a nearly complete structure. The problem arises when it is necessary to provide local reinforcement for joints and cutouts that are always present in aerospace structures.
Typically it is desirable to discontinue the ribs or stringers in the local vicinity of the joint or cutout so they do not interfere or complicate the design of the hardware associated with molded joints and also to provide smooth introduction of loads into the structure.
In addition, it is necessary to increase the thickness of the structure in the joint region due to the concentrated loads being introduced and due to the stress concentrations created by bolt holes. This requirement for local reinforcement is complicated by two interrelated factors. First, composite materials are inherently anisotropic (they have different mechanical properties in different directions in the plane of a laminate). In order to design efficient and failure resistant joints in composite materials, fibers must be oriented in multiple directions (relative to the loading direction). When fibers are dispersed in all directions, the resulting laminate is termed "pseudo-isotropic".
A second factor that complicates local joint reinforcement is the fact that filament winding cannot provide localized reinforcement of fiber orientations of other than 90.degree. degrees because filament winding is a process that produces structure by continuously wrapping or winding a bundle of fibers around a mandrel (similar to winding a ball of string). Therefore the "local" reinforcement in the joint region must be done either by and lay-up or by secondary assembly operations. Both methods significantly increase the recurring costs of these structures.
It is an object of the invention to provide a novel isogrid concept that produces low cost joints for filament wound composites in the aerospace industry.
It is also an object of the invention to provide a novel isogrid concept for automatically providing reinforced joints that eliminate material scrappage, secondary operations and reduces part count.
It is another object of the invention to provide a novel isogrid concept that overcomes the problem of high cost associated with providing for joints in typical filament wound isogrid structures.